In the field of fluid heating systems for domestic, commercial and industrial use of heated fluid, there are fluid-tank heaters and tankless fluid heaters. For example, water-tank heating systems, such as those found in many homes throughout the world, provide hot water by heating a large volume of water in a water tank. This is wasteful since such a large volume of hot water is needed only intermittently. Tankless water heaters seek efficiency by heating water on demand. Typically, in a tankless system, heat is concentrated about a section of conduit through which the water flows from the water source to the water use point. The section of conduit may be coiled to allow more water to be heated at a time as the water passes through the region of space heated by the heat source. The heat source may be electrical, flame, or microwave.
There are many types of tankless water heater systems and microwave water heaters described in the art. For example U.S. Pat. No. 5,387,780 discloses a “microwave powered boiler” for heating water. A first cabinet is provided that surrounds and protects a second cabinet made of a material such as steel that reflects microwave energy. In the interior space between the first and second cabinet is a thermal insulating material. Enclosed within the second cabinet is a third cabinet that forms a tank where the water is heated. A microwave source coupled to wave guiding structure feeds microwave energy to the region between the second and third cabinet. The wall of the third cabinet allows microwave energy to penetrate there through to heat the water enclosed thereby. A thermostat control system is provided so that when the water temperature in the tank is lower than the set point, the magnetron microwave source is initiated to generate microwave energy to heat the water until the set point is reached.
An example of a tankless water heater system that uses a microwave source and a coiled conduit section is provided by Electro Silica, a provider of water heating systems. Their website on the World Wide Web is electrosilica.com. There is shown a system wherein cold water received from a water source flows generally downward through a coiled conduit section that is enclosed within a stainless steel tank. The coil is disposed against the interior wall of the tank. Above the coil and above or at the top of the region enclosed by the tank, is a set of magnetrons that produce microwave energy at 2450 Mega-Hertz (MHz). The coil is flexible and made of a silica-based substance that enables microwaves to penetrate there through and heat the water therein. The metal tank shaped, purportedly to prevent “generation of refraction and diffraction waves.” The base of the chamber formed by the tank serves as a reflecting dish to direct energy upward towards the silica based flexible coil in the chamber. As demand for water is made, the magnetron sources initiate to produce microwave energy that propagates in the chamber. Some of this energy propagates into the water in the coiled conduit and is absorbed by the water to generate heat. At the bottom of the steel chamber is an outlet to supply water that has been heated within the chamber.
One problem presented by tankless and microwave heaters is the relative inefficiency of energy transfer. Ideally, one would want all of the generated energy to be converted to heat only the fluid as it passes through a well-defined region. In practice, some energy generated by the source will not heat the fluid, but rather, will be dissipated and conducted away by structure exterior to the fluid. In the case of microwave-heated systems, some microwave energy never enters the water, but is reflected away by the boundary of the water-carrying conduit section. This further reduces efficiency.
For at least these reasons, there is a need for a more efficient electromagnetic-energy-heated tankless water heater.